The present invention is directed to devices useful for polarizing the coherent light output of infrared lasers. Polarization and polarization devices broadly, and particularly in the visible region of the electromagnetic spectrum, have been known for many years. See, for example, chapter 24 of a book entitled, "Fundamentals of Optics", by Frances A. Jenkins and Harvey E. White, published by the McGraw-Hill Book Company in 1957. For a description of the particular Brewster's Angle Polarizer being discussed herein reference is made to FIG. 24d on page 491 of that volume. For an alternate point of view, reference is also made to chapter 2 of a book entitled, "Introduction to Modern Optics", written by Grant R. Fowles and published by Holt, Reinhart, and Winston, Inc. in 1968.
As will seen by the above, polarizing devices for use in the visible wavelength region of the spectrum have been made quite efficient. That is to say, single crystal polarizers can be fabricated which have very high polarization extinction ratios; where that term is here taken to mean the ratio of the intensity of polarized light transmitted when the polarizers are aligned to the intensity of the same input light transmitted when the polarizers are crossed. This is not so easily achieved in the infrared wavelength region, however, with devices of any practical size or complexity.
One approach which has recently been taken toward infrared polarizers is the so-called "wire grid" polarizer. This device is described, for example, in an article by G. R. Bird et al entitled, "The Wire Grid as a Near Infrared Polarizer", published in the Journal of Optical Society of America, Vol. 50, No. 9, September 1960, beginning at page 886. See also an article by P. A. Cheo and C. D. Bass entitled, "Efficient Wire-Grid Duplexer Polarizer for CO.sub.2 Lasers", published in Applied Physics Letters, Vol. 18, No. 12, on 15 June 1971, beginning at page 565, and also an article by Hugh L. Garvin entitled "High Resolution Fabrication by Ion Beam Sputtering", published in the magazine, Solid State Technology, in November 1973 beginning at page 31.
The wire grid polarizer is an array of closely-spaced parallel wires supported by a transmitting substrate. For wavelengths significantly shorter than the spacing of the wires, radiation with an E vector parallel to the wires will be reflected. Due to reflection from the surfaces of the supporting substrates, the reflected beam will contain a mixture of E (parallel) and E (perpendicular) radiation. These wire grid polarizers may be obtained from PTR Optics, Corp., 145 Newton St., Waltham, Mass. 02154. This company deals in infrared laser optics and their literature states that the wire grid polarizers produced by PTR are constructed by ruling a finally blazed diffraction grating on a precisely polished zinc selenide substrate. The ruled area is then coated with aluminum vacuum deposited at an oblique angle to form highly conductive wires at the peaks of the grooves. The 15 mm square polarizers are mounted in 25 mm diameter anodized aluminum holders. For a device having 1200 lines per millimeter on a zinc selenide substrate, the extinction ratio is stated to be greater than 40 to 1 at 2.5 microns radiation wavelength, greater than 100 to 1 at 3 microns, greater than 400 to 1 at 4 microns, and greater than 1000 to 1 at 10 microns.
In many practical applications, one desires higher extinction ratios than those quoted. Also one wishes to have an arrangement which eliminates the back reflected component which might otherwise be returned to the laser source and degrade modulation or other measured effects while at the same time providing an output beam which is in the same projected line or direction as the input beam to the polarizer and has thus not been deviated by passage through the polarizer even under conditions of rotation thereof.
It is an object of this invention to provide a polarizing arrangement which can achieve each of the foregoing characteristics simultaneously.